Adult patients with an acute ischemic stroke seen within six hours of the onset of symptoms will be randomized to receive either placebo or two different doses of aptiganel HCl (an NMDA antagonist - neuroprotective agent) to be given as a bolus followed by a 12-hour infusion. Patients not requiring ICU admission for their stroke will be admitted to the GCRC for study drug infusion, cardiac monitoring, pharmacokinetic blood draws, and other standard stroke therapy. Since approval of tissue plasminogen activator (TPA) as a standard therapy for patients seen during the first three hours of a stroke, agents such as NMDA antagonists will hopefully provide salvage for patients with stroke symptoms of longer duration.